[GHC] #14332: Deriving clauses can have forall types

[GHC]

#14332: Deriving clauses can have forall types
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.2.1
  checker)                           |
      Resolution:                    |             Keywords:  deriving
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 OK, that's a start. But I have more questions:

 * Are the `forall`'d type variables in a `deriving` clause's type assumed
 to be distinct from the type variables bound by a data type itself? If so,
 how should GHC treat derived instances that would only work if kind
 unification were to occur? For instance, `data Proxy (a :: k) = Proxy
 deriving (Generic1 :: (k -> Type) -> Constraint)` is clearly OK, but `data
 Proxy (a :: k) = Proxy deriving (forall k. Generic1 :: (k -> Type) ->
 Constraint)` isn't, since it can't unify the `k` from the derived type and
 the `k` from the datatype. What should GHC do here?
 * What happens if one of the variables isn't quantified over (e.g., `data
 D = D deriving (forall a. C a b)`)? Is this an error?

 Many I'd be convinced if I did see a full write-up of this in a proposal.
 But as it stands, I'm quite unconvinced that this would work out if you
 actually sat down and attempted to implement this.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14332#comment:4>
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